10.3.1 Character Definition Arrays

Each simple character set has a configuration file located in
the sql/share/charsets directory. For a
character set named MYSYS, the file
is named
MYSET.xml. It
uses <map> array elements to list
character set properties. <map>
elements appear within these elements:

<ctype> defines attributes for each
character.

<lower> and
<upper> list the lowercase and
uppercase characters.

<unicode> maps 8-bit character
values to Unicode values.

<collation> elements indicate
character ordering for comparisons and sorts, one element
per collation. Binary collations need no
<map> element because the character
codes themselves provide the ordering.

For a complex character set as implemented in a
ctype-MYSET.c
file in the strings directory, there are
corresponding arrays:
ctype_MYSET[],
to_lower_MYSET[],
and so forth. Not every complex character set has all of the
arrays. See also the existing ctype-*.c
files for examples. See the
CHARSET_INFO.txt file in the
strings directory for additional
information.

Most of the arrays are indexed by character value and have 256
elements. The <ctype> array is indexed
by character value + 1 and has 257 elements. This is a legacy
convention for handling EOF.

<ctype> array elements are bit values.
Each element describes the attributes of a single character in
the character set. Each attribute is associated with a bitmask,
as defined in include/m_ctype.h:

The <ctype> value for a given character
should be the union of the applicable bitmask values that
describe the character. For example, 'A' is
an uppercase character (_MY_U) as well as a
hexadecimal digit (_MY_X), so its
ctype value should be defined like this:

ctype['A'+1] = _MY_U | _MY_X = 01 | 0200 = 0201

The bitmask values in m_ctype.h are octal
values, but the elements of the <ctype>
array in
MYSET.xml should
be written as hexadecimal values.

The <lower> and
<upper> arrays hold the lowercase and
uppercase characters corresponding to each member of the
character set. For example:

lower['A'] should contain 'a'
upper['a'] should contain 'A'

Each <collation> array indicates how
characters should be ordered for comparison and sorting
purposes. MySQL sorts characters based on the values of this
information. In some cases, this is the same as the
<upper> array, which means that sorting
is case-insensitive. For more complicated sorting rules (for
complex character sets), see the discussion of string collating
in Section 10.3.2, “String Collating Support for Complex Character Sets”.